1. Field of the Invention
The present invention relates to an image forming apparatus and an image formation control method thereof, which are applied to improve image quality when electrophotographic image formation is performed.
2. Description of the Related Art
Conventionally, an image forming apparatus comprises an image adjustment function that all the adjustment process is operated in the device in order to suppress changes in color concomitantly to a change in environment or a change over time, and changes in color after replacement of each part due to consumption and various types of maintenance work (see e.g. Japanese Laid-Open Patent Publication (Kokai) No. H09-319270 and Japanese Laid-Open Patent Publication (Kokai) No. H09-326926).
The above related art has executed the following control. The control includes maximum density (Dmax) control, tone (half tone) control, detection of life duration of various parts, and toner density control in a developing unit. The control is executed when a certain condition is reached based on counter information indicating the printing (copying) number of sheets, power source information indicating the state of a power source, and environmental sensor (temperature-humidity sensor) information indicating the state of environment; thus allowing the image to be stabilized.
The above tone control is control by which image density is detected by a sensor placed on a photoconductive drum or a transfer belt, or on the downstream side of a fixing device, and the detected image density is compared with target image density and corrected. The tone control can suppress changes in color occurring due to a change in environment or a change over time, and has applications using an image forming apparatus as a proofer (color proofreader).
In addition, there is an increase in applications using the image forming apparatus as a convenience printer, a so-called print on demand (POD) machine, concomitantly to an increase in speed. In particular, an electrophotographic image forming apparatus can instantly form images without proofing operation, in recent years drawing attention as a printer suitable for individual image formation according to a client's demand, a so-called variable data printing.
Meanwhile, a technique has been proposed where the frequency of image density adjustment is adjusted (controlled) by predicting the state of the image forming apparatus (see e.g. Japanese Laid-Open Patent Publication (Kokai) No. H10-186769).
However, it has been known that when an electrophotographic image forming apparatus is used for image formation, the fluctuation level of the image density is different depending on the application and frequency in use of the image forming apparatus, in particular, depending on difference (level) in printing rates. This is caused by variations in the amount of supply of a developer accompanying difference in the consumption of developers depending on difference in printing rates and variations in the rising property of the amount of electrical charge of the supplied developer.
In particular, when individual image formation is performed in response to the demand of a client, such as in the above variable data printing, since the printing rate is extremely different from one copy of a print product to another, a problem arises that a common image color does not match at each copy of the print product, or the image color does not match for a predetermined portion. Therefore, there is a problem that a good image with matching color cannot be provided to each client, and the purpose of the variable data printing cannot be achieved sufficiently.
In order to solve the above problem, an approach of more frequently adjusting image density is considered. However, when the image density is frequently adjusted, problems arise that a developer for adjusting the image density is excessively consumed, and productivity is reduced. In addition, the excessively consuming the developer causes the problem to worsen when a print product in which a shift in color does not pose much of a problem is formed, for example, even when an image is formed for flyers for simple distribution, excessive adjustment of image density is executed.
Meanwhile, when a technique of adjusting (controlling) the frequency of image density adjustment described in the above Japanese Laid-Open Patent Publication (Kokai) No. H10-186769 is applied to an image forming apparatus with a high copy number of image formation per unit time such as a convenience printer (POD machine), which causes the following problem. Even if image density is adjusted after the image density changed, a predetermined time is needed until the image density is adjusted, that is to say, until feedback is applied. As a result, there is a problem that it is impossible to stabilize the color of the image in a predetermined range.